The present invention relates to a system for precise control over mechanical linkages, and more particularly to an apparatus for the micro-manipulation of links in a robotic system which overcomes the effects of simple and complex direct-drive nonlinearities.
Several manufacturing tasks require robots or hard-automated machines that are capable of very fine manipulations. In the assembly of optical fibers, for example, the required accuracy is on the order of 3 to 10 micrometers. Another application is in the assembly of magnetic cores for computers, where typical part dimensions are less than 1.0 millimeter and the assembly tolerancing error is on the order of 0.25 micrometer. Other recent applications such as integrated circuit manufacturing requires positioning stages with accuracies as low as 0.025 micrometer.
These applications require much higher precision in the design and control of robot manipulators than are currently available. The direct-drive approach in manipulator design has succeeded somewhat in improving the performance of robot manipulator systems by eliminating many problems inherent in gear systems, such as backlash, friction due to meshing, and mechanical compliance. Prior art direct-drive systems, however, exhibit some drawbacks. First, since the torque produced by the motor is used directly to drive the output link, the drive system must provide all of the necessary torque, requiring larger direct-drive motors and power amplifiers. Furthermore, having a one-to-one transmission ratio is disadvantageous in that disturbances and unmodelled dynamics of the manipulator arm are reflected onto the drive system shaft, making it difficult to control. Additional drawbacks that limit performance, particularly in micro-manipulation, are the nonlinearities in the amplifier-motor part of the drive system. Since no gear reducer is used, these system nonlinearities are transmitted directly to the output shaft and thereby interfere with accurate control.
The problem of compensating for nonlinearities has always been an important issue in control engineering. Most of the prior art solutions are based on control techniques such as adaptive control or nonlinear controller design using inverse describing functions. While such techniques may bring about solutions that deal with some of the more common nonlinearities, they are limited to controlling the electrical signal supplied to the direct-drive motor, rather than the motor torque output, and there still remain some problems associated with their implementation and effectiveness. In most cases an accurate and correct model of the nonlinearity is needed.
In general, drive system nonlinearities can be divided into two groups: simple nonlinearities and complex nonlinearities. Simple nonlinearities include single-valued, piece-wise linear nonlinearities such as dead band, coulomb friction, and preload. Complex nonlinearities include multi-valued nonlinearities such as backlash in a gear train or hysteresis in the electromagnetics of a motor. The existence of these nonlinearities, whether simple or complex, presents a serious control problem that can degrade both the transient and steady-state characteristics of system response.
Typically, in a robot manipulator drive system there are two major sources for both groups of nonlinearity, the actuator and the transmission. Transmission nonlinearities include compliance of a cable mechanism or spline in a harmonic drive, backlash, friction due to gear meshing, preload in bearings, etc. Also, various actuators have some inherent nonlinear characteristics such as medium compressibility in hydraulic or pneumatic systems, the dead band in power amplifiers, and hysteresis in the electromagnetics of an electric motor.
The transmission nonlinearities have been effectively compensated for in the prior art, using techniques such as spring-loaded antibacklash mechanisms resulting in "backlashless" gears. Unfortunately, these mechanisms also increase the friction considerably, making accurate manipulation difficult. Yet, to date there has been little or no effort in attempting to compensate for the nonlinear effects in actuator systems. The main reason for this is simply that there has not been a substantial need for it. The robots of the prior art have been applied generally to tasks that require only "moderate" precision, such as spray-painting, pick-and-place of low tolerance materials, and welding. However, as robots become applied to higher-precision manipulation tasks, undesirable behavior of actuator dynamics must be well compensated for.
Direct-drive actuator systems offer many advantages in precision applications over systems with gear trains or linkages, but three major nonlinearities associated with the motor-amplifier combination of a direct-drive system still remain: power stage electronics dead zone, torque ripple, and electromagnetic hysteresis.
The effect of power stage electronics dead zone appears in the output voltage of the amplifier. For a typical system, the dead zone may be as large as one volt in either direction. The large dead zone comes mainly from a time delay in switching the power transistors in the amplifier from positive to negative voltage output, and vice versa. The time delay is used to guarantee that the output voltage settles to zero before the next switching occurs.
Dead zone effect in the amplifier depends largely on the design of the particular amplifier. In some of the newer versions of power amplifiers, an output voltage bias is added to minimize these effects. Another prior art approach is use of feedforward compensation, which requires an accurate model of the nonlinearity. If an accurate model of dead zone can be obtained, such nonlinear compensation can greatly minimize the effect. However, one drawback of this method may be in actual implementation of such nonlinear controller, as an accurate model is difficult to obtain.
The second direct-drive nonlinearity is torque ripple which arises from variations in actual motor torque output as a function of the angle of the rotor. This can typically be mathematically modelled by a superposition of two different sinusoids. The period of the first sinusoid corresponds to the number of magnetic poles around the rotor. The second sinusoid is characterized by a period that corresponds to the number of conductors around the stator. Once the model is obtained, the feedforward compensation technique can be employed to minimize or eliminate such nonlinear effect. Both dead zone and torque ripple are simple nonlinearities which have been dealt with, to some extent, by the prior art.
Electromagnetic hysteresis exists in the motor-amplifier system, and is primarily a problem with permanent magnet DC motors which are widely used in direct-drive systems. There are three main sources that contribute to electromagnetic hysteresis nonlinearly in a DC motor. The first one is what is often referred to as iron core loss. The cross product of the permanent magnet field and the electric field in the winding is the torque produced by the motor. When the current in the winding is turned off, there is still a small amount of residual field in the same direction as the electric field. This arises due to the remaining field in the iron core after the current is turned off.
The second source is the eddy current effect in the winding. The third is a linear B-H relationship that exists in the air gap. When the air gap effect is combined with the nonlinear hysteretic effect from the iron core loss and the eddy current loss, a complex electromagnetic hysteresis results. Generally, the absolute width of the hysteresis is small, and this kind of complex nonlinearity has been neglected in the prior art for all practical purposes. However, it may have a significant effect when microrange fine manipulation is desired.
There is no known prior art actuator system that can completely eliminate the effect of a complex nonlinearity such as electromagnetic hysteresis. The nature of a complex nonlinearity is that there are two output values for any given input. This makes it very difficult for any conventional control action to fully eliminate its effect.
It is therefore a principal object of the present invention to provide a system which permits the micro-manipulation of robotic systems while overcoming complex nonlinearities such as electromagnetic hysteresis.
A further object of the present invention is to provide a robotic manipulation system which also overcomes simple nonlinearities such as torque ripple and dead zone effect.
Yet a further object is to provide a system that is relatively inexpensive and easy to construct.
Other objects will in part be apparent and in part pointed out hereinafter.